This invention relates to piezoelectric transducers that are responsive to changing mechanical forces applied thereto. The changing mechanical forces may be produced by respiratory or cardiac activity of patients.
The piezoelectric transducer is made of material which produces an electrical charge that is proportional to the degree of strain in the piezoelectric material due to the motion of a mass exerting a force thereon. An electrical potential develops along certain crystallographic lattice axes of the material in response to movement of charge as a result of mechanical deformation of the material. The crystal lattice structure of the material is physically deformed by application of an increasing force caused by the moving mass. The deformation of the lattice produces a relative displacement of the positive and negative charges within the crystal lattice internal to the material. The displacement of the internal charges produce equal external charges of opposite polarity on the opposite surfaces of the material creating the "piezoelectric effect". The charges may be measured by applying metal contacts or electrodes to the opposite surfaces of the piezoelectric material and measuring the potential difference between them. The magnitude and polarity of the induced surface charges are proportional to the magnitude and direction of the applied force, produced by the moving mass, as given by: EQU Q(coulombs)=d(coulomb/m.sup.2 /newton/m.sup.2)F(newtons/m.sup.2)
where Q is the surface charge, d is the piezoelectric constant and F is the applied force.
The piezoelectric transducer may be considered electrically equivalent to a charge generator, delivering a charge proportional to the force applied to the piezoelectric material. The piezoelectric transducer may be connected to a charge-to-voltage converter so that the electrical charge provided to the converter is proportional to the rate of change of the force applied or deformation of the transducer.
It is possible to induce the "piezoelectric effect" in certain synthetic resin polymers (organic compounds) by cooling them from a liquid or soft state to the solid state in the presence of an electric field or by polymerization in the presence of an electric field. Typical materials used to make such piezoelectric transducers (electret transducers) are beeswax and polymers such as polyvinylidene fluoride. For example, U.S. Pat. No. 3,792,204 issued to Murayana et al. shows a transducer composed of a piezoelectric film of a polyvinylidene fluoride resin having electroconductive material on the opposite surfaces of the film. The molecules of the film are oriented by stretching the film. The "piezoelectric effect" is produced when a force is applied perpendicular to the plane of the film causing deformation of the film parallel to the direction of molecular orientation. The transducer uses the electret of a vinylidene fluoride resin film as a vibrator or oscillator to provide for an acoustic transducer, U.S. Pat. No. 3,996,922, which is a divisional of U.S. Pat. No. 3,898,981, issued to Basham shows a force responsive transducer that senses changes due to reciprocating forces and motions caused by respiration or heart rate. The force responsive electret transducer has movable parts adapted for placement beneath a patient or a patient support such as a mattress. There is no direct attachment of the transducer to the patient. The protective covering material for the transducer is vinyl, the electrodes attached to the electret material are flexible steel sheets and the electret film is Teflon. The normal breathing motion of the patient produces a varying applied force against the electrodes. The increasing force causes the electrodes to be moved nearer the electrically polarized film in a manner similar to moving a conductor through an electric field such that a current flows through a conductor connecting the electrodes to electronic circuitry. The current flow in the conductor varies with the force applied to the electrode which, in turn, varies with the patient's respiration rate.
With the application of a high (polarizing) voltage to produce the electric fields for inducing the "piezoelectric effect" in suitable materials, there is a reorientation of the crystalline structure which persists after removal of the polarizing voltage. Often this induction process is carried out at an elevated temperature. This technique, in addition to producing a material with a high piezoelectric constant, removes the geometrical constraints of crystallographic axes and makes it possible to cast piezoelectric materials in any desired form.
A piezoelectric material need be distorted only a small amount to obtain a voltage in the fractional volt range. For this reason, piezolectric material may be considered as efficient isometric transducers. The stiffness of piezoelectric materials is usually high, and the permissible deformations are small. For example, the deformation of the crystal material used in phonograph pickups is ten (10) microns per gram of weight.
An output voltage cannot be maintained by the piezoelectric material when a sustained force is applied. Therefore, piezoelectric material is generally suited to the measurement of changing mechanical forces. Piezoelectric material can produce an output voltage for changes in mechanical deformation having a frequency of a few Hertz to many megaHertz. The upper frequency limit is determined by the total mass and stiffness of the moving transducer.
U.S. Pat. No. 4,144,877 issued to Frei et al. shows piezoelectric electret transducers made of organic compounds which are formed by cooling from the liquid or soft state to the solid state in the presence of an electric field or by polymerization in the presence of an electric field. Typical materials used to make electrets or beeswax and polymers such as polyvinylidene fluoride. Conducting electrodes are deposited on the electret material by known means and can be in an array configuration.
U.S. Pat. No. 4,204,135 issued to Murayana shows organic piezoelectric elements in which the shifting of the piezoelectric modulus by the influence of stretching conditions is not as large for an unoriented vinylidene fluoride copolymer as for a vinylidene fluoride homopolymer.
Other pertinent patents include U.S. Pat. No. 3,820,529 issued to Gause et al. and U.S. Pat. No. 4,299,233 issued to Lemelson.
It is an object of this invention to monitor both respiratory and cardiac activity of a patient using a transducer with a synthetic resin polymer electret.
An object of this invention is to provide a transducer, responsive to changing mechanical forces, that operates in a relatively low frequency bandwidth.
Another object of this invention is to provide for a transducer having both high sensitivity to changing mechanical forces and noise immunity.
Other objects of the invention include providing a transducer that is not connected directly to the patient, that is easy to use and that can be adapted for use by adults as well as children and that gives an accurate indication of both heart and respiratory activity regardless of the patient's position on the transducer.